Initial commit: notification-elements-demo app

Interactive Angular 19 demo for @sda/notification-elements-ui with
6 sections: Bell & Feed, Notification Center, Inbox, Comments &
Threads, Mention Input, and Full-Featured layout. Includes mock
data, dark mode toggle, and real-time event log.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

node_modules/autoprefixer/lib/hacks/flex-spec.js

@@ -0,0 +1,19 @@
+/**
+ * Return flexbox spec versions by prefix
+ */
+module.exports = function (prefix) {
+  let spec
+  if (prefix === '-webkit- 2009' || prefix === '-moz-') {
+    spec = 2009
+  } else if (prefix === '-ms-') {
+    spec = 2012
+  } else if (prefix === '-webkit-') {
+    spec = 'final'
+  }
+
+  if (prefix === '-webkit- 2009') {
+    prefix = '-webkit-'
+  }
+
+  return [spec, prefix]
+}